The year ahead
When the festivals fall
A festival is a position in the sky, not a date on a calendar: Diwali is the Kartika new moon and Janmashtami is the eighth of Bhadrapada's dark fortnight. The day each lands on is whatever the sky says that year — so these are computed, not copied.
How these dates are worked out
The lunar month is named by the sidereal sign the Sun stands in at the new moon that opens it — Sun in Karka gives Shravana, Sun in Simha gives Bhadrapada, round the twelve.
North India reckons a month from full moon to full moon, so the dark fortnight carries the NEXT month's name. The same day is Shravana Krishna Tritiya in Pune and Bhadrapada Krishna Tritiya in Delhi. These dates use the northern reckoning, and the position is printed beside each one so there is nothing to guess.
A festival's day is the day whose SUNRISE its tithi is running at — with three exceptions that are kept at a different hour and are marked as such. Diwali is one: the Kartika new moon can be running at dawn on a day when it has already gone by dusk, and Lakshmi puja is at dusk. That is a one-day difference and it is the commonest error in a computed calendar.
The festivals are the sky's calendar. What is your own?
The same engine reads your birth moment — every house, every graha, every period with dates.